Guidance for interfaces

MEF is working like a charm, there is only one area I'm a wee bit fuzzy on and was wondering what the official guidance will be. Does the host application distribute an interface library for third-parties to develop to/with? Note:
my ILogger interface references enum parameters which are also in the interface library.

In the my proof of concept (fully functional, code
HERE) I have three solutions: DebugLogger, MessageBoxLogger and the main application BasicWPFApp. All three applications reference the same MEFContrib.Library.Interface project. Both DebugLogger and MessageBoxLogger projects are
configured to only copy their respective .DLL's to the Extensions folder (not the ComponentModel or MEFContrib.Library.Interface dlls which they have a reference to).

Full source for the BaseWPFApp, minus using statements and xaml, follows below (Bootstrapper is in a MEFContrib.Library project):